For convenience, the following abbreviations are herewith defined, at least some of which are referred to within the following description of the state-of-the-art and the present invention.    CVoIP Carrier VoIP    DSL Digital Subscriber Line    GW Gateway    IMS IP Multimedia Subsystem    IP Internet Protocol    PC Personal Computer    PSAP Public Safety Answering Point    PSTN Public Switched Telephone Network    RTP Real-time Transport Protocol    SIP Session Initiation Protocol    TA Terminal Adaptor    VoIP Voice over Internet Protocol
In a traditional PSTN, subscribers would be provided with telephones for use in a fixed location, such as a home or business. The telephone service provider would run a wire from its network to the premises, where it would typically terminate at a junction box. Another wire could then be run from the junction box to the telephone. Sometimes a circuit would connect two or more telephones to the junction box, allowing all of them to use the same telephone line to the provider network. The additional phones are traditionally called extension telephones. When the telephone rang, any or all of them could be used to answer the call, which will be heard on any telephone that is off-hook. An outgoing call could also be placed using any extension, though when a call is in process, it captured the line and no other calls can be initiated. In most modern installations, a number of telephone ports positioned throughout the premises are similarly connected in a circuit to the junction box, so that a telephone equipped with a suitable jack can be plugged in and used at any one of them. Of course, more than one telephone may be plugged in at one time, and if so they will operate the same as the extensions that are hardwired to the circuit.
A PSTN is a hierarchical network of switches through which a call made on the telephone line establishes a communication path to the called party. Each telephone converts the conversation into analog signals that are carried back and forth through this circuit and converted into audible conversation for the other party. This circuit between the calling party and the called party remains in place until the communication session is terminated, when the network resources, that is, the individual switches and interconnecting wires or cables, are freed so that they could be used for another telephone call.
More recently, the Internet and similar packet data networks have been used as an alternative to the existing PSTN. In this type of communication, a VoIP telephone converts the conversation into digital data packets that are transported through the network to the called party. The VoIP telephone may be an instrument designed for this use, or may be a regular analog telephone used in conjunction with a TA. Through the network, the packets are routed to a called party with a VoIP telephone that can convert the received packets back into an audible conversation. The packets may also be routed to a media gateway connected to a PSTN. The gateway is able to convert the digital packets into an analog signal communication session so that the call may be terminated at a regular analog telephone as well. The structure of this communication system will be described in more detail below.
VoIP subscribers may have extension telephones as well, so long as each telephone is able to access the network. Since the telephones do not reside on the same analog circuit, however, extension phone service is typically operated somewhat differently than with the PSTN. When a call for the location arrives, all extension telephones are contacted separately, and a communication session is established with the first one that answers; in this case the other extensions may not participate in the call.
Service providers offer a number of ancillary services in addition to simply enabling telephone calls between two parties. One popular service is caller ID, in which the calling party's telephone number is determined and transmitted to the called party along with the signal that causes their phone to ring. Equipped with an appropriate instrument, the called party is the able to view the number, and sometimes the identity of the caller if the system is so enabled, even before the call is accepted.
Naturally, subscribers may use caller ID to decide whether to accept an incoming call, but the service has also found other uses. Emergency numbers, such as 911, are directed to PSAPs near where the caller is located. The telephone service provider is aware of the address of the subscriber as well as their phone number, and so they provide this information to the PSAP along with the caller's telephone number. As should be apparent, this can be of great value in emergency situations, especially where the caller is not physically able to communicate effectively.
With VoIP services, however, a problem dealing with emergency service calls may arise. This is because VoIP telephones may be installed in devices like laptop computers, which are frequently carried around by a subscriber when they travel. The subscriber may then place telephone calls wherever network access is available. If the “nomadic” telephone is configured as an extension to the subscriber's home telephone, it will ring when the subscriber's number is dialed, and allow them to answer the call regardless of their location. For many uses, this may be quite advantageous; the subscriber may in this way simply use their home telephone number regardless of where they are located. The caller ID feature still operates, that is, parties called from the VoIP telephone are still presented with the caller's home number. While this is desirable in a number of circumstances, it frustrates operation of the emergency response system.
When a subscriber places an emergency call using a nomadic extension telephone, their home caller ID information is used when routing the call. A PSAP provided with this information may access a location database to look up the associated address and, if the subscriber is unable to communicate effectively, the emergency response will be dispatched to the home address. Not only does the subscriber fail to get the needed assistance, but the emergency crews sent to the wrong location will be tied up and unavailable for a genuine emergency in their area. For these reasons, in some areas emergency calls may not be permitted from VoIP extension telephones.
Accordingly, there has been and still is a need to address the aforementioned shortcomings and other shortcomings associated with emergency call service availability for nomadic VoIP extension telephones. These needs and other needs are satisfied by the present invention.